docs: def _extract_feature(fun, image, mask = slice(None), **kwargs):
      """
      Convenient function to cope with multi-spectral images and feature normalization.
      
      Parameters
      ----------
      fun : function
          The feature extraction function to call
      image : array_like or list/tuple of array_like 
          A single image or a list/tuple of images (for multi-spectral case).
      mask : ndarray
          The binary mask to select the voxels for which to extract the feature
      kwargs : sequence
          Additional keyword arguments to be passed to the feature extraction function 
      """
      if not type(mask) is slice:
          mask = numpy.array(mask, copy=False, dtype=numpy.bool)
      
      if type(image) == tuple or type(image) == list:
          return join(*[fun(i, mask, **kwargs) for i in image])
      else:
          return fun(image, mask, **kwargs)